Bench coach Alan Trammell is almost certainly not a candidate to replace Gibson; the two are closely aligned, and viewed as virtually indistinguishable from one another, sources say.

The D-Backs' Triple-A manager, former major leaguer Phil Nevin, joined the organization only this season after managing the Tigers' Triple-A affiliate for three years.

Another potential candidate, D-Backs broadcaster Bob Brenly, led the club to the 2001 World Series title in his first season, but hasn't managed since '04.

As for the front office, none of Towers' special assistants are viewed as likely successors. General managing partner Ken Kendrick, in a recent interview with the Arizona Republic, indicated that he wants the team to rely more on data, and a GM with that type of background almost certainly would come from outside the organization.

Replacing a GM at mid-season generally is considered more difficult than replacing a manager. Teams are in the middle of preparing for the amateur draft the first week of June, and after that will turn their attention to the non-waiver trade deadline on July 31.